To permit a check of the authenticity of bank notes, a multiplicity of apparatuses have been proposed. It has become known, for example, to detect feature substances present in bank notes by means of a sensor in order to check the authenticity of the particular bank note. As a rule, the bank notes are illuminated for this purpose with light of a certain wavelength. The light of the certain wavelength excites the feature substances to themselves emit light of another wavelength, so-called luminescent light. The light remitted by the bank notes is detected by a detector and the bank notes are recognized as authentic if the detector detects the luminescent light.
With known apparatuses it is readily possible to check the authenticity of the bank notes, but known apparatuses also offer starting-points for forgers to ascertain without any great effort how the check of authenticity of the bank notes is carried out. This only requires for example determining the wavelength of the illumination as well as the wavelength of the light remitted by the bank notes. The wavelengths then permit conclusions to be drawn on the feature substances used in the bank notes.
Known apparatuses for checking the authenticity of bank notes thus have the disadvantage of leading upon their use—up to a certain extent—to an unmasking of the feature substances used for securing the authenticity of the bank notes. This is particularly problematic when the apparatuses are used in unprotected areas, e.g. in places where they are accessible to a broad group of people. Such apparatuses are in particular relatively small bank note processing machines for counting, accounting for and evaluating bank notes, but also testing devices for accepting and/or dispensing bank notes.